# Light Lenses.

## Presentation on theme: "Light Lenses."— Presentation transcript:

Light Lenses

Revision (refraction)
Refraction occurs when the light ray changes mediums. Light traveling through air and then going through water is an example of a light ray changing medium. The speed of the light ray changes when it enters a different medium. In most cases the direction of the light also changes. We say the light bends.

Revision (refraction)
FST = Fast to Slow, Towards Normal. If a ray of light passes across the boundary from a material in which it travels fast into a material in which travels slower, then the light ray will bend towards the normal line.

Revision (refraction)
SFA = Slow to Fast, Away From Normal. If a ray of light passes across the boundary from a material in which it travels slow into a material in which travels faster, then the light ray will bend away from the normal line.

What is a Lens? A lens is a transparent material, such as glass, that has either one curved surface and one flat surface or two curved surfaces. When light travels through lenses, refraction occurs. The light bends either outward or inward, it depends on the lens.

Why use a lens? As a lens alters the path of light rays, they are used to assist humans in many ways. Each of us have convex lenses’ in our eyes. These are to focus the light coming from objects around us, onto the back of our eye (the retina).

Types of Lenses Convex lens (or converging lens): Due to refraction, light rays bend as they pass into and out of the lens. Convex lenses are shaped so that the rays converge together. Concave lens (or diverging lens): concave lenses are shaped to spread rays apart.

Anatomy of a Lens [1] If a symmetrical lens is thought of as being a slice of a sphere, then there would be a line passing through the center of the sphere and attaching to the mirror in the exact center of the lens. This imaginary line is known as the principal axis.

Anatomy of a Lens [2] A lens also has an imaginary vertical axis which bisects the symmetrical lens into halves.

Anatomy of a Lens [3] light rays traveling parallel to the principal axis will either converge or diverge. If the light rays converge (convex), then they will converge to a point. This point is known as the focal point of the converging lens. If the light rays diverge (concave), then the diverging rays can be traced backwards until they intersect at a point. This intersection point is known as the focal point of a diverging lens

Anatomy of a Lens [4] The focal point is denoted by F. Note that each lens has two focal points - one on each side of the lens. Lenses can allow light to pass through either face, depending on where the incident rays are coming from. Subsequently, every lens has two possible focal points. A lens also has an imaginary point which we refer to as the 2F point. This is the point on the principal axis which is twice as far from the vertical axis as the focal point is.

Image Formation Refraction Rules for a Converging Lens: Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens. Any incident ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis. An incident ray which passes through the center of the lens will in affect continue in the same direction that it had when it entered the lens. First lets consider a double convex lens. Suppose that several rays of light approach the lens; and suppose that these rays of light are traveling parallel to the principal axis. Upon reaching the front face of the lens, each ray of light will refract towards the normal to the surface. At this boundary, the light ray is passing from air into a more dense medium (usually plastic or glass). Since the light ray is passing from a medium in which it travels fast (less optically dense) into a medium in which it travels relatively slow (more optically dense), it will bend towards the normal line. This is the FST principle of refraction. This is shown for two incident rays on the diagram below. Once the light ray refracts across the boundary and enters the lens, it travels in a straight line until it reaches the back face of the lens. At this boundary, each ray of light will refract away from the normal to the surface. Since the light ray is passing from a medium in which it travels slow (more optically dense) to a medium in which it travels fast (less optically dense), it will bend away from the normal line; this is the SFA principle of refraction.

Image Formation Refraction Rule for a Diverging Lens: Any incident ray traveling parallel to the principal axis of a diverging lens will refract through the lens and travel in line with the focal point (i.e., in a direction such that its extension will pass through the focal point). Any incident ray traveling towards the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis. An incident ray which passes through the center of the lens will in affect continue in the same direction that it had when it entered the lens. Now let's investigate the refraction of light by double concave lens. Suppose that several rays of light approach the lens; and suppose that these rays of light are traveling parallel to the principal axis. Upon reaching the front face of the lens, each ray of light will refract towards the normal to the surface. At this boundary, the light ray is passing from air into a more dense medium (usually plastic or glass). Since the light ray is passing from a medium in which it travels relatively fast (less optically dense) into a medium in which it travels relatively slow (more optically dense), it will bend towards the normal line. This is the FST principle of refraction. This is shown for two incident rays on the diagram below. Once the light ray refracts across the boundary and enters the lens, it travels in a straight line until it reaches the back face of the lens. At this boundary, each ray of light will refract away from the normal to the surface. Since the light ray is passing from a medium in which it travels relatively slow (more optically dense) to a medium in which it travels fast (less optically dense), it will bend away from the normal line. This is the SFA principle of refraction. These principles of refraction are identical to what was observed for the double convex lens above.

Convex lens (Ray trace diagram)
1. Pick a point on the top of the object and draw three incident rays traveling towards the lens. Using a straight edge, accurately draw one ray so that it passes exactly through the focal point on the way to the lens. Draw the second ray such that it travels exactly parallel to the principal axis. Draw the third incident ray such that it travels directly to the exact center of the lens. Place arrowheads upon the rays to indicate their direction of travel.

Convex lens (Ray trace diagram)
2. Once these incident rays strike the lens, refract them according to the three rules of refraction for converging lenses. The ray that passes through the focal point on the way to the lens will refract and travel parallel to the principal axis. Use a straight edge to accurately draw its path. The ray which traveled parallel to the principal axis on the way to the lens will refract and travel through the focal point. And the ray which traveled to the exact center of the lens will continue in the same direction. Place arrowheads upon the rays to indicate their direction of travel. Extend the rays past their point of intersection.

Convex lens (Ray trace diagram)
3. Mark the image of the top of the object. The image point of the top of the object is the point where the three refracted rays intersect. All three rays should intersect at exactly the same point. This point is merely the point where all light from the top of the object would intersect upon refracting through the lens. Of course, the rest of the object has an image as well and it can be found by applying the same three steps to another chosen point.

Convex lens (Ray trace diagram)
4. Repeat the process for the bottom of the object. One goal of a ray diagram is to determine the location, size, orientation, and type of image which is formed by the double convex lens. Typically, this requires determining where the image of the upper and lower extreme of the object is located and then tracing the entire image. After completing the first three steps, only the image location of the top extreme of the object has been found. Thus, the process must be repeated for the point on the bottom of the object. If the bottom of the object lies upon the principal axis (as it does in this example), then the image of this point will also lie upon the principal axis and be the same distance from the mirror as the image of the top of the object. At this point the entire image can be filled in.

Concave Lens (Ray trace diagram)
1. Pick a point on the top of the object and draw three incident rays traveling towards the lens. Using a straight edge, accurately draw one ray so that it travels towards the focal point on the opposite side of the lens; this ray will strike the lens before reaching the focal point; stop the ray at the point of incidence with the lens. Draw the second ray such that it travels exactly parallel to the principal axis. Draw the third ray to the exact center of the lens. Place arrowheads upon the rays to indicate their direction of travel.

Concave Lens (Ray trace diagram)
2. Once these incident rays strike the lens, refract them according to the three rules of refraction for double concave lenses. The ray that travels towards the focal point will refract through the lens and travel parallel to the principal axis. Use a straight edge to accurately draw its path. The ray which traveled parallel to the principal axis on the way to the lens will refract and travel in a direction such that its extension passes through the focal point on the object's side of the lens. Align a straight edge with the point of incidence and the focal point, and draw the second refracted ray. The ray which traveled to the exact center of the lens will continue to travel in the same direction. Place arrowheads upon the rays to indicate their direction of travel. The three rays should be diverging upon refraction.

Concave Lens (Ray trace diagram)
3. Locate and mark the image of the top of the object. The image point of the top of the object is the point where the three refracted rays intersect. Since the three refracted rays are diverging, they must be extended behind the lens in order to intersect. Using a straight edge, extend each of the rays using dashed lines. Draw the extensions until they intersect. All three extensions should intersect at the same location. The point of intersection is the image point of the top of the object. The three refracted rays would appear to diverge from this point. This is merely the point where all light from the top of the object would appear to diverge from after refracting through the double concave lens. Of course, the rest of the object has an image as well and it can be found by applying the same three steps to another chosen point.

Concave Lens (Ray trace diagram)
4. Repeat the process for the bottom of the object. The goal of a ray diagram is to determine the location, size, orientation, and type of image which is formed by the double concave lens. Typically, this requires determining where the image of the upper and lower extreme of the object is located and then tracing the entire image. After completing the first three steps, only the image location of the top extreme of the object has been found. Thus, the process must be repeated for the point on the bottom of the object. If the bottom of the object lies upon the principal axis (as it does in this example), then the image of this point will also lie upon the principal axis and be the same distance from the lens as the image of the top of the object. At this point the complete image can be filled in. Some students have difficulty understanding how the entire image of an object can be deduced once a single point on the image has been determined. If the object is merely a vertical object (such as the arrow object used in the example below), then the process is easy. The image is merely a vertical line. This is illustrated in the diagram below. In theory, it would be necessary to pick each point on the object and draw a separate ray diagram to determine the location of the image of that point. That would require a lot of ray diagrams. Due to this we stick to simple straight line objects to ray trace.

Image Location Convex lenses can produce real images which can be caught on a screen or a virtual image which cannot be caught on a screen. Concave lenses produce only virtual images

Image Location (terminology)
Upright – The same way up as the original object. Inverted – the opposite way up to the original object. Enlarged – bigger than the original object. Diminished – smaller than the original image. Magnification – the image size divided by the size of the original object

Uses - telescope If you had a "bigger eye," you could collect more light from the object and create a brighter image, and then you could magnify part of that image so it stretches out over more pixels on your retina. Two pieces in a telescope make this possible: The objective lens (in refractors) or primary mirror (in reflectors) collects lots of light from a distant object and brings that light, or image, to a point or focus. An eyepiece lens takes the bright light from the focus of the objective lens or primary mirror and "spreads it out" (magnifies it) to take up a large portion of the retina. This is the same principle that a magnifying glass (lens) uses; it takes a small image on the paper and spreads it out over the retina of your eye so that it looks big.

Uses - Microscopes A microscope uses the same trick as a refracting telescope — light waves being bent as they travel through glass. In a telescope, the idea is to bend parallel light from very faraway objects into a small focus at the eye. In a microscope, the idea is to bend diverging (spreading-out) light into a parallel path, then bend that parallel-path light into a small focus at the eye.

binoculars are great for seeing things on Earth and in the sky
binoculars are great for seeing things on Earth and in the sky. Suppose you see a strange light on the horizon. Have the flying saucers finally arrived from Alpha Centauri? You grab your binoculars and take a look. When you do, the light from the UFO encounters the objective lenses (one on each side) of the binoculars. Just as in a telescope or a microscope, the glass lens bends the light by slowing it down. The bent light beams race through the body of the binoculars, bouncing off the two prisms before they pass through the eyepiece and enter your eyes. The size and placement of the lenses determine just how magnified the image is when it enters your eyes. For instance, binoculars rated 7 x 50 make the image seven times larger. The 50 is the size of the objective lens, measured in millimeters. The larger the objective lehttp://images.google.com.au/imgres?imgurl=http://www.comparestoreprices.co.uk/images/bu/bushnell-natureview-binoculars-8x42.jpg&imgrefurl=http://www.comparestoreprices.co.uk/binoculars/bushnell-natureview-binoculars-8x42.asp&usg=__9y73FIZn5OjkjSiOQY-2wQppB8M=&h=287&w=350&sz=16&hl=en&start=9&tbnid=DlvKdrimfLoBMM:&tbnh=98&tbnw=120&prev=/images%3Fq%3Dbinoculars%26hl%3Den%26safe%3Doff%26sa%3DGns, the more light it gathers from the object.

Student Work Complete the questions on pages